But by nightfall on Thursday, what had been a search for food took a more menacing turn, as groups of looters, some of them armed, swooped in and took whatever of value was left: electronics, appliances and vehicles.

“All the food is gone now,” Jacques Charbonnier, a 63-year-old resident of St. Martin, said in an interview on Sunday. “People are fighting in the streets for what is left.”

In the few, long days since Irma pummeled the northeast Caribbean, killing more than two dozen people and leveling 90 percent of the buildings on some islands, the social fabric has begun to fray in some of the hardest-hit communities.